Overall Survival [ Time Frame: During study period (up to 3 years) ] [ Designated as safety issue: No ]

Overall survival was defined as the time from randomization to death due to any cause. For patients who were alive at the time of the analysis data cutoff, overall survival was censored at the last date the patient was known to be alive or analysis data cutoff date, whichever was first. This included patients who were known to have died after the data analysis cutoff date. Patients with no post-baseline survival information were censored on the date of randomization.

Radiographic progression-free survival was defined as the time from randomization to the first objective evidence of radiographic disease progression assessed by independent central radiology review or death due to any cause within 168 days after treatment discontinuation, whichever was first. Radiographic disease progression was evaluated by CT scan or MRI and radionuclide bone scans at regularly scheduled visits. Radiographic disease progression in bone required a confirmatory scan. Radiographic disease progression in soft tissue did not require a confirmatory scan for purposes of analysis. Radiographic disease progression was evaluated by independent central radiology review using RECIST 1.1 for soft tissue disease and PCWG2 guidelines for bone disease. Patients who did not reach the endpoint were censored at their last assessment.

Time to First Skeletal-related Event [ Time Frame: During study period (up to 3 years) ] [ Designated as safety issue: No ]

Time to first skeletal-related event was defined as the time from randomization to the date of the first occurrence of a skeletal-related event for each patient. A skeletal-related event was defined as radiation therapy or surgery to bone for prostate cancer, pathological bone fracture, spinal cord compression, or initiation/change in antineoplastic therapy to treat bone pain from prostate cancer. Skeletal-related events were recorded at each scheduled and unscheduled study visit and during long-term follow-up if a skeletal-related event was not documented previously. Patients who did not have a skeletal-related event at the time of the analysis data cutoff were censored at the date of last assessment indicating no evidence of skeletal-related event. Patients with no postbaseline assessments were censored on the date of randomization.

Time to Initiation of Cytotoxic Chemotherapy [ Time Frame: During study period (up to 3 years) ] [ Designated as safety issue: No ]

The time to initiation of cytotoxic chemotherapy is defined as the time from randomization to the date of initiation of cytotoxic chemotherapy for the treatment of prostate cancer for each patient. For patients who did not start cytotoxic chemotherapy at the time of the analysis data cutoff, time to initiation of cytotoxic chemotherapy was censored at the date of last assessment where no cytotoxic chemotherapy was indicated or at the analysis data cutoff date, whichever was first. Time to initiation of cytotoxic chemotherapy for patients with no postbaseline assessments was censored on the date of randomization.

Time to Prostate-specific Antigen (PSA) Progression [ Time Frame: During study period (up to 3 years) ] [ Designated as safety issue: No ]

Time to PSA progression was defined as the time from randomization to date of first confirmed observation of PSA progression for each patient. For patients with PSA declines at week 13, the PSA progression date was defined as the date that a ≥ 25% increase and an absolute increase of ≥ 2 ng/mL above the nadir was documented, and confirmed 3 or more weeks later. For patients with no PSA decline at week 13, the PSA progression date was defined as the date that a ≥ 25% increase and an absolute increase of ≥ 2 ng/mL above baseline was documented, and confirmed 3 or more weeks later. For patients who did not have confirmed PSA progression at the time of the analysis data cutoff, time to PSA progression was censored at the date of the last PSA assessment showing no evidence of confirmed PSA progression or the analysis data cutoff date, whichever was first. Time to PSA progression for patients with no postbaseline assessments was censored on the date of randomization.

PSA response was defined as a ≥ 50% reduction in PSA from baseline to the lowest postbaseline PSA value and required confirmation by a consecutive assessment at least 3 weeks later. Patients were evaluable for PSA response rate if a patient had a PSA level measured at baseline and at least one postbaseline assessment.

Best Overall Soft Tissue Response [ Time Frame: During study period (up to 3 years) ] [ Designated as safety issue: No ]

The best overall soft tissue objective response is defined as partial response [PR] or complete response [CR] while on study treatment based on investigator assessments of target, nontarget, and new lesions using RECIST 1.1. Soft tissue was assessed by CT or MRI at regularly scheduled visits. Only patients with measurable soft tissue disease (ie, at least 1 target lesion identified per RECIST 1.1) at screening are included in this analysis. All percentages are based on number of participants with measurable soft tissue disease at screening in each treatment group.

The purpose of this study is to determine the benefit of enzalutamide versus placebo as assessed by overall survival and progression-free survival in patients with progressive metastatic prostate cancer who have failed androgen deprivation therapy but not yet received chemotherapy.

Participants received enzalutamide 160 mg, administered as four 40-mg capsules, once per day by mouth. Study drug treatment continued until disease progression (evidence of radiographic progression, a skeletal-related event, or clinical progression) and the initiation of a cytotoxic chemotherapy or an investigational agent, unacceptable toxicity, or withdrawal.

Drug: Placebo

Participants received placebo, administered as four capsules, once per day by mouth. Study drug treatment continued until disease progression (evidence of radiographic progression, a skeletal-related event, or clinical progression) and the initiation of a cytotoxic chemotherapy or an investigational agent, unacceptable toxicity, or withdrawal.